Characterization of Solidification Microstructures in Vacuum Arc Remelted Nickel Alloy 718

Digital image analysis techniques were developed to autonomously characterize dendritic solidification features in microstructures of a nickel alloy 718 ingot produced by vacuum arc remelting (VAR). Automated macrophotography was implemented to image microstructures and produce large image montages of etched ingot cross sections. Two image analysis techniques, particle identification and two-point correlation, were applied to autonomously measure primary dendrite arm orientation and secondary dendrite arm spacing from the image montages. Particle identification measured individual dendritic features. The two-point correlation technique measured averaged feature values over defined image areas. These methods are described and compared. Melt pool profile and solidification time histories were reconstructed from primary dendrite arm orientation and secondary dendrite arm spacing measurements, respectively. These characterization techniques provide two new methods of reconstructing melt pool profiles and solidification times in cast ingots. The information produced is expected to be useful in the validation of computational models for solidification during VAR, similar remelting processes, and general casting processes that produce dendritic solidification features.